1. Field of the Invention
This invention relates, generally, to reinforced silica composites and a method of making the same. More particularly, the invention relates to a graphite fiber reinforced silica composite having an extremely low thermal expansion coefficient and to articles fabricated from such composites, such as laser mirrors, which exhibit high temperature dimensional stability.
2. Description of the Related Art
The aerospace industry has long recognized the advantages of composite materials of construction, particularly those which exhibit superior physical properties, such as low density combined with high temperature dimensional stability. One of the most promising materials for use in composite construction is graphite fiber, such as high mechanical strength, high elastic modulus graphite fiber yarn. Although such graphite yarn has heretofore been used in the formation of useful structural composites, the need for composites having high temperature dimensional stability has continued to be unmet. This need is illustrated in the manufacture of high energy laser mirrors.
High energy lasers, such as powerful gas lasers, have an excited medium which provides the lasing action. Aligned with the excited medium is a pair of optical resonator mirrors between which light oscillations occur. Precision mirrors used as resonator mirrors are constructed of an inert substrate such as molybdenum or single crystal silicon upon the surface of which is applied a mirror coating having a high degree of radiation reflectivity. Effective laser action depends upon a buildup of energy by repeated reflection of radiation between the laser mirrors prior to escaping as a high energy coherent beam through one of the mirrors. In practice, imperfect alignment of the mirrors impairs the development or maintenance of the proper oscillation of the reflected radiation, which misalignment is frequently due to dimensional changes, i.e. distortion of the mirror optical surface, due to the large thermal energy absorbed at the surface during laser operation. Each mirror in the optical path which is used to transmit the laser beam is subject to the same distortion. Thus, it is necessary to minimize the distortion of all mirrors in a high energy laser system in order to reduce the distortion of the wavefront of the laser beam.
To prevent mirror distortion, complex heat exchangers are utilized in the mirror substrate to remove the absorbed energy and minimize distortion of the optical surface. Mirror distortion may also be reduced by employing a high heat transfer coefficient between the cooling medium and the heat exchanger passages in combination with or as an alternative to utilizing a substrate material which exhibits good thermal conductivity and an extremely low, near zero, coefficient of thermal expansion. For example, one major factor of laser mirror distortion may be expressed mathematically by the following equation:
Mirror distortion ##EQU1## where Q/A=absorbed flux intensity
.alpha.=mirror coefficient of thermal expansion PA1 h=heat transfer coefficient PA1 t=thickness between optical surface and coolant passages PA1 k=mirror thermal conductivity
By reference to the mathematical expression above, it can be seen that reduction of the coefficient of thermal expansion (.alpha.) provides a greater reduction in mirror distortion than can be achieved by any practical increase in heat transfer coefficient (h) or increased thermal conductivity. The coefficient of thermal expansion plays an even greater role in the two remaining components of mirror distortion, namely heat exchanger bending strain and mirror support structure bowing.
Unalloyed molybdenum and single crystal silicon have been used as substrate materials for laser mirrors because of their relatively low coefficients of thermal expansion, e.g. .alpha.=2.7.times.10.sup.-6 /.degree.F. and .alpha.=1.70.times.10.sup.-6 /.degree.F. (from 20.degree. C. to 150.degree. C.) respectively, good thermal conductivity and high modulus of elasticity. However, the performance of molybdenum and single crystal silicon as substrate materials is limited by their fixed coefficient of thermal expansion and molybdenum is further limited by its relatively high density.